1. Field of the Invention
The present invention relates to an X-ray imaging apparatus having at least one automatic exposure control (AEC) pixel for performing real-time detection for part of an X ray that is made incident via a subject. The X-ray imaging apparatus performs AEC based on a signal detected by the AEC pixel, and takes an X-ray image of the subject, based on the X-ray.
2. Description of the Related Art
A known X-ray imaging apparatus will be described with reference to FIG. 4, which depicts an X-ray tube 101, a patient 102, an imaging unit 103, a grid 104, an AEC detector 112, an X-ray detector 105, a CPU 106, and an X-ray generator 107.
When an X-ray-generation signal transmitted from the CPU 106 is turned on, the X-ray generator 107 makes the X-ray tube 101 operate so that the X-ray tube 101 generates an X-ray. The X-ray emitted from the X-ray tube 101 passes through the patient 102 as a subject and reaches the imaging unit 103. At that time, the X-ray is scattered and absorbed in the body of the patient 102. Since only primary radiation traveling straight into the body of the patient 102 can be used for imaging, scattered radiation is unnecessary for the X-ray detector 105.
Generally, the grid 104 is used for removing the unnecessary scattered radiation to improve the contrast of the X-ray image. The AEC detector 112 is used for decreasing the quantity of X-ray exposure received by a human body as much as possible and for adjusting the X-ray imaging apparatus such that a predetermined amount of X-rays are emitted. An AEC system including the above-described AEC detector 112 is often referred to as a “phototimer”.
The degree of scattering and absorption of the X-ray in the patient 2 changes according to the composition of the body of the patient 102. Where the body of the patient 102 is thick, the degree of scattering and absorption is high. On the other hand, if the body of the patient 102 is thin, the degree of scattering and absorption is low. However, in comparing two patients with the same body thickness but different amounts of muscle or fat, the degree of scattering and absorption of the patients will differ.
Further, the degree of the absorption also changes according to the body part exposed to the X-ray. For example, in the case where a hand or a leg, which is thin, is exposed to the X-ray, only a small amount of scattered radiation is generated. In this case, the grid 4 is preferably removed to decrease the exposure dose.
The amount of X-rays that are made incident to the AEC detector 112 changes according to the grid state, such as whether or not the grid is mounted, the grid type, and exposure conditions affecting a grid image. Therefore, it has been difficult to correctly estimate the amount of primary radiation passing through the patient 2 before exposure, based on the amount of the X-rays that are made incident to the AEC detector 12.